Post your links that might be helpful for the development of games based in space - whether it is for art, procedural generation or just informations about our universe. I'll keep the original post updated as more people post their links, and try to categorize them. I'll start with mines.

The NASA's Bluemarble ftp site (direct link): This is the ULTIMATE link if you're looking for high-resolution Earth datasets. The ftp site is slow to connect but works. You can get heightmaps, texture maps, cloud maps, normal maps and light maps of the full Earth at a resolution of 1 km (43200x21600 texels).

Celestia is a free real-time space simulation that lets you experience our universe in three dimensions. Unlike most planetarium software, Celestia doesn't confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy. All travel in Celestia is seamless; the exponential zoom feature lets you explore space across a huge range of scales, from galaxy clusters down to spacecraft only a few meters across. A 'point-and-goto' interface makes it simple to navigate through the universe to the object you want to visit.

I dont have any links right now, but I like this idea for a thread, and I'd just like to suggest that if you're planning to update the original post with the additional links that you somehow mark the latest additions to save people reading through the whole list to find the new links.

one thing I never understood about Star Trek is the 'USS' in front of every ship's name. so.... what does it stand for ? United States Ship maybe? yeeeessssss, of course. Why not painting an US Flag on every ship?

Very well done IMO. Unfortunately it's Windows-only, and requires a very powerful CPU and GPU. You can explore the entire known universe it seems, complete with billions of galaxies, stars, planets, etc. with a combination of real and procedurally generated data.

Also, I'd like to plug my own project here (if that's okay) in case anyone finds it useful: It's not an astronomically correct simulation, but it's a 100% open-source 3D universe engine in WebGL/HTML5. It supports trillions of stars and procedurally generated (concurrent, so everything is seamless) planets. IT does not support: atmospheric scattering, galaxy visualization, nebulae, ground level detail like cities/forests -- however all of these are major goals I want to accomplish with Kosmos 2.0 (my next spare time project).

I post this here in case any of the code from Kosmos is helpful to others working on space-related 3D engines, as I solved a lot of misc. problems during its development that are common to large space game environments (128 bit coordinate systems, layered rendering due to zbuffer precision issues, planet terrain LOD, etc.) Kosmos doesn't have super high resolution planets though because it's targeted at laptops (not extremely powerful GPUs), but I'm working on a more efficient engine in C/C++/OpenGL in my spare time that hopefully should allow cities, forests, etc. for ground level detail. (I found WebGL/JavaScript to be too performance limiting, though still impressive how far browsers have progressed so far.)